1. Technical Field of the Invention
The present invention relates to a composition, a polymer and a tilt angle controlling agent that are useful for producing an optically anisotropic layer, a retardation plate produced by using the same, and a process for producing the retardation plate. The invention also relates to a liquid crystal display device having the retardation plate.
2. Related Art
A liquid crystal display device generally comprises a first polarizing plate and a second polarizing plate provided with a liquid crystal cell intervening therebetween, and the liquid crystal cell has a pair of substrates and a liquid crystal layer containing a rod-like liquid crystal compound intervening therebetween. It has been known in the art that birefringence occurring in the liquid crystal cell employing a rod-like liquid crystal compound is compensated with an optical compensation sheet having an optically anisotropic layer formed with a discotic liquid crystal compound (such as 2,3,6,7,10,11-hexa(4-(4-acryloyloxyhexyloxy)benzoyloxy)-triphenylene). In this case, there are some cases where the phase difference cannot be compensated over the entire wavelength range of light due to difference in wavelength dispersibility between the rod-like liquid crystal compound and the discotic liquid crystal compound to cause discoloration (such as failure in black color).
A trisubstituted benzene compound substituted with a heterocyclic group has also been reported as the discotic liquid crystal compound (in Macromolecular Crystals and Liquid Crystals, vol. 370, p. 391 (2001). However, it is difficult to attain low wavelength dispersibility, i.e., making the wavelength dispersion close to constant, by using the compound, and such a compound is demanded that has smaller wavelength dispersibility (i.e., a small value of (Re of short wavelength (e.g., 450 nm))/(Re of long wavelength (e.g., 650 nm))).
While the difference in wavelength dispersibility has been mainly discussed herein, the retardation value Re(λ) of the retardation plate is also important. It is necessary to determine the retardation Re(λ) of the retardation plate corresponding to the optical property of the liquid crystal cell to be compensated. The retardation (Δn·d) is the product of the refractive index anisotropy (Δn) of the optically anisotropic layer and the thickness (d) of the optically anisotropic layer. In the case where the optically anisotropic layer has a larger refractive index anisotropy (Δn), the liquid crystal cell can be compensated even with the layer having a smaller thickness (d). In a retardation plate produced by fixing the orientation of liquid crystals, the retardation (Re) varies depending on the orientation angle (i.e., the tilt angle and the mean tilt angle) of the oriented liquid crystal, and it is therefore necessary to control the orientation angle.
JPA No. 2002-129162 proposes a compound having a ring structure represented by the specific formula as a liquid crystal orientation accelerator. JPA No. 2006-16599 proposes a polymerizable liquid crystal composition containing a (meth)acrylate copolymer (H) having a side chain containing a fluorine group and a side chain having a group containing a ring structure represented by the specific formula.
However, these conventional orientation controlling agents do not exhibit orientation controlling capability to any liquid crystal compound, and are insufficient as an orientation controlling agent, for example, for the aforementioned tri-substituted benzene type discotic liquid crystal compound substituted with a heterocyclic group. In particular, it is difficult to subject the tri-substituted benzene type discotic liquid crystal compound substituted with a heterocyclic group to hybrid orientation with a low mean tilt angle (for example, 40° or less), and an orientation controlling agent capable of controlling such an orientation state is demanded.
In the case where an optically anisotropic film or the like is produced by curing a liquid crystal molecule through polymerization or the like, it is demanded that the tilt angle is not changed upon fluctuation in temperature on curing, from the standpoint of production stability of the optically anisotropic film or the like. However, although the tilt angle of the liquid crystal molecule can be controlled with the conventional orientation controlling agent, the temperature dependency of the tilt angle thereof is large to deteriorate the production stability, and improvement thereof is demanded.